1. Field of the Invention
The present invention relates to power hand tools, in particular to impact tools, such as impact wrenches.
2. Description of the Prior Art
Impact wrenches for applying intermittent torque impulses to tighten or loosen a fastener are well known. These prior mechanisms include a rotatable drive shaft connected to a motor, a hammer disposed about and coupled to the shaft, and an anvil engageable with a load, either directly or by means of a coupling tool, such as a socket. The anvil has a pair of ears engageable with corresponding ears on the hammer. These impact mechanisms also include a spring for biasing the hammer toward the anvil to engage the hammer ears with the anvil ears and a cam mechanism to allow the hammer to rotate with respect to the shaft and to move axially along the shaft away from the anvil when resistance torque builds up at the workpiece. In a reversible impact wrench, the cam mechanism commonly includes a pair of V-shaped cam ramps or grooves on the exterior of the drive shaft and a corresponding pair of cam ramps or grooves on the interior surface of the hammer and two balls respectively disposed in the grooves. Each leg of each V-shaped groove defines a portion of a helix. When the drive shaft rotates clockwise and enough resistance torque is built up, the cam mechanism causes the hammer to move axially away from the anvil as the balls travel along one respective leg of the V of each groove. When the shaft rotates counterclockwise, the ball travels along the other respective leg of the V of each groove as the hammer retracts axially. Depending on the torque build up, the hammer can retract axially a great enough distance such that the ball bottoms out at the end of the groove of the drive shaft. This can cause the end surfaces of the grooves and the balls to be worn and roughened, making it more difficult for the motor to rotate the shaft, and may require the replacement of either the balls or shaft.